Abstract

This specification defines a mechanism by which user agents may verify that a fetched resource has been delivered without unexpected manipulation.

Status of This Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

This document has been reviewed by W3C Members, by software developers, and by other W3C groups and interested parties, and is endorsed by the Director as a W3C Recommendation. It is a stable document and may be used as reference material or cited from another document. W3C's role in making the Recommendation is to draw attention to the specification and to promote its widespread deployment. This enhances the functionality and interoperability of the Web.

W3C expects the
functionality specified in this Recommendation will not be
affected by changes to Fetch. The Working Group will continue to track the Fetch
specification and document issues that impact this specification.

1. Introduction

This section is non-normative.

Sites and applications on the web are rarely composed of resources from only a single origin. For example, authors pull scripts and styles from a wide variety of services and content delivery networks, and must trust that the delivered representation is, in fact, what they expected to load. If an attacker can trick a user into downloading content from a hostile server (via DNS poisoning, or other such means), the author has no recourse. Likewise, an attacker who can replace the file on the Content Delivery Network (CDN) server has the ability to inject arbitrary content.

Delivering resources over a secure channel mitigates some of this risk: with
TLS, HSTS, and pinned public keys, a user agent can be fairly certain that it is indeed speaking with the server it believes it’s talking to. These mechanisms, however, authenticate only the server, not the content. An attacker (or administrator) with access to the server can manipulate content with impunity. Ideally, authors would not only be able to pin the keys of a server, but also pin the content, ensuring that an exact representation of a resource, and only that representation, loads and executes.

This document specifies such a validation scheme, extending two HTML elements with an integrity attribute that contains a cryptographic hash of the representation of the resource the author expects to load. For instance, an author may wish to load some framework from a shared server rather than hosting it on their own origin. Specifying that the expected SHA-384 hash of
https://example.com/example-framework.js is Li9vy3DqF8tnTXuiaAJuML3ky+er10rcgNR/VqsVpcw+ThHmYcwiB1pbOxEbzJr7 means that the user agent can verify that the data it loads from that URL matches that expected hash before executing the JavaScript it contains. This integrity verification significantly reduces the risk that an attacker can substitute malicious content.

This example can be communicated to a user agent by adding the hash to a
script element, like so:

Scripts, of course, are not the only response type which would benefit from integrity validation. The scheme specified here also applies to link and future versions of this specification are likely to expand this coverage.

1.1 Goals

Compromise of a third-party service should not automatically mean compromise of every site which includes its scripts. Content authors will have a mechanism by which they can specify expectations for content they load, meaning for example that they could load a
specific script, and not any script that happens to have a particular URL.

The verification mechanism should have error-reporting functionality which would inform the author that an invalid response was received.

1.2 Use Cases/Examples

1.2.1 Resource Integrity

An author wishes to use a content delivery network to improve performance for globally-distributed users. It is important, however, to ensure that the CDN’s servers deliver only the code the author expects them to deliver. To mitigate the risk that a CDN compromise (or unexpectedly malicious behavior) would change that site in unfortunate ways, the following
integrity metadata is added to the link element included on the page:

An author wants to include JavaScript provided by a third-party analytics service. To ensure that only the code that has been carefully reviewed is executed, the author generates integrity metadata for the script, and adds it to the script element:

A user agent wishes to ensure that JavaScript code running in high-privilege HTML contexts (for example, a browser’s New Tab page) aren’t manipulated before display.
Integrity metadata mitigates the risk that altered JavaScript will run in these pages’ high-privilege contexts.

2. Conformance

As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.

The key words MAY, MUST, and SHOULD are to be interpreted as described in [RFC2119].

Conformance requirements phrased as algorithms or specific steps can be implemented in any manner, so long as the end result is equivalent. In particular, the algorithms defined in this specification are intended to be easy to understand and are not intended to be performant. Implementers are encouraged to optimize.

2.1 Key Concepts and Terminology

This section defines several terms used throughout the document.

The term digest refers to the base64-encoded result of executing a cryptographic hash function on an arbitrary block of data.

2.2 Grammatical Concepts

WSP (white space) characters are defined in Section
2.4.1 Common parser idioms of the HTML 5 specification as
White_Space characters.

3. Framework

The integrity verification mechanism specified here boils down to the process of generating a sufficiently strong cryptographic digest for a resource, and transmitting that digest to a user agent so that it may be used to verify the response.

3.1 Integrity metadata

To verify the integrity of a response, a user agent requires integrity
metadata as part of the request. This metadata consists of the following pieces of information:

For example, given a script resource containing only the string alert(\'Hello, world.\');, an author might choose SHA-384 as a hash function.
H8BRh8j48O9oYatfu5AZzq6A9RINhZO5H16dQZngK7T62em8MUt1FLm52t+eX6xO is the base64-encoded digest that results. This can be encoded as follows:

3.2 Cryptographic hash functions

User agents SHOULD refuse to support known-weak hashing functions like MD5 or SHA-1 and SHOULD restrict supported hashing functions to those known to be collision-resistant. Additionally, user agents SHOULD re-evaluate their supported hash functions on a regular basis and deprecate support for those functions that have become insecure. See Hash collision attacks.

3.2.1 Agility

Multiple sets of integrity metadata may be associated with a single resource in order to provide agility in the face of future cryptographic discoveries. For example, the resource described in the previous section may be described by either of the following hash expressions:

When a hash function is determined to be insecure, user agents SHOULD deprecate and eventually remove support for integrity validation using the insecure hash function. User agents MAY check the validity of responses using a digest based on a deprecated function.

To allow authors to switch to stronger hash functions without being held back by older user agents, validation using unsupported hash functions acts like no integrity value was provided (see the “Does response match metadataList” algorithm below). Authors are encouraged to use strong hash functions, and to begin migrating to stronger hash functions as they become available.

3.2.2 Priority

User agents must provide a mechanism for determining the relative priority of two hash functions and return the empty string if the priority is equal. That is, if a user agent implemented a function like getPrioritizedHashFunction(a,
b) it would return the hash function the user agent considers the most collision-resistant. For example, getPrioritizedHashFunction('sha256',
'sha512') would return 'sha512' and getPrioritizedHashFunction('sha256',
'sha256') would return the empty string.

Note

The getPrioritizedHashFunction is an internal implementation detail. It is not an API that implementors provide to web applications. It is used in this document only to simplify the algorithm description.

3.3 Response verification algorithms

3.3.1 Apply algorithm to response

Let result be the result of applying algorithm to the representation data without any content-codings applied, except when the user agent intends to consume the content with content-encodings applied. In the latter case, let result be the result of applying algorithm to the representation data.

Let encodedResult be result of base64-encoding
result.

Return encodedResult.

3.3.2 Is response eligible for integrity validation

In order to mitigate an attacker’s ability to read data cross-origin by brute-forcing values via integrity checks, responses are only eligible for such checks if they are same-origin or are the result of explicit access granted to the loading origin via Cross Origin Resource Sharing [CORS].

Note

As noted in RFC6454, section 4, some user agents use globally unique identifiers for each file URI. This means that resources accessed over a file scheme URL are unlikely to be eligible for integrity checks.

Note

Being in a Secure Context (e.g., a document delivered over HTTPS) is not necessary for the use of integrity validation. Because resource integrity is only an application level security tool, and it does not change the security state of the user agent, a Secure Context is unnecessary. However, if integrity is used in something other than a Secure Context (e.g., a document delivered over HTTP), authors should be aware that the integrity provides no security
guarantees at all. For this reason, authors should only deliver integrity metadata in a Secure Context. See Non-secure contexts remain non-secure for more discussion.

which would allow the user agent to accept two different content payloads, one of which matches the first SHA384 hash value and the other matches the second SHA384 hash value.

Note

User agents may allow users to modify the result of this algorithm via user preferences, bookmarklets, third-party additions to the user agent, and other such mechanisms. For example, redirects generated by an extension like
HTTPS Everywhere could load and execute correctly, even if the HTTPS version of a resource differs from the HTTP version.

Note

This algorithm returns false if the response is not eligible for integrity validation since Subresource Integrity requires CORS, and it is a logical error to attempt to use it without CORS. Additionally, user agents SHOULD report a warning message to the developer console to explain this failure.

3.4 Verification of HTML document subresources

A variety of HTML elements result in requests for resources that are to be embedded into the document, or executed in its context. To support integrity metadata for some of these elements, a new integrity attribute is added to the list of content attributes for the link and script elements.

A corresponding integrity IDL attribute which reflects the value each element’s integrity content attribute is added to the
HTMLLinkElement and HTMLScriptElement interfaces.

Note

A future revision of this specification is likely to include integrity support for all possible subresources, i.e., a, audio, embed, iframe, img,
link, object, script, source, track, and video elements.

3.5 The integrity attribute

The integrity attribute represents integrity metadata for an element. The value of the attribute MUST be either the empty string, or at least one valid metadata as described by the following ABNF grammar:

The integrity IDL attribute must reflect the integrity content attribute.

option-expressions are associated on a per hash-expression basis and are applied only to the hash-expression that immediately precedes it.

In order for user agents to remain fully forwards compatible with future options, the user agent MUST ignore all unrecognized option-expressions.

Note

Note that while the option-expression has been reserved in the syntax, no options have been defined. It is likely that a future version of the spec will define a more specific syntax for options, so it is defined here as broadly as possible.

3.6.1.1 Attributes

3.6.2 HTMLScriptElement

3.6.2.1 Attributes

3.7 Handling integrity violations

The user agent will refuse to render or execute responses that fail an integrity check, instead returning a network error as defined in Fetch [FETCH].

Note

On a failed integrity check, an error event is fired. Developers wishing to provide a canonical fallback resource (e.g., a resource not served from a CDN, perhaps from a secondary, trusted, but slower source) can catch this
error event and provide an appropriate handler to replace the failed resource with a different one.

3.8 Elements

3.8.1 The link element for stylesheets

Whenever a user agent attempts to obtain a resource pointed to by a
link element that has a rel attribute with the keyword of stylesheet, modify step 4 to read:

Do a potentially CORS-enabled fetch of the resulting absolute URL, with the mode being the current state of the element’s crossorigin content attribute, the origin being the origin of the link element’s Document, the default origin behavior set to taint, and the integrity metadata of the request set to the value of the element’s integrity attribute.

3.8.2 The script element

Let src be the value of the element’s src attribute and the request’s associated integrity metadata be the value of the element’s
integrity attribute.

4. Proxies

Optimizing proxies and other intermediate servers which modify the responses MUST ensure that the digest associated with those responses stays in sync with the new content. One option is to ensure that the integrity metadata associated with resources is updated. Another would be simply to deliver only the canonical version of resources for which a page author has requested integrity verification.

To help inform intermediate servers, those serving the resources SHOULD send along with the resource a Cache-Control header with a value of no-transform.

5. Security Considerations

This section is non-normative.

5.1 Non-secure contexts remain non-secure

Integrity metadata delivered by a context that is not a Secure Context, such as an HTTP page, only protects an origin against a compromise of the server where an external resources is hosted. Network attackers can alter the digest in-flight (or remove it entirely, or do absolutely anything else to the document), just as they could alter the response the hash is meant to validate. Thus, it is recommended that authors deliver integrity metadata only to a
Secure Context. See also securing the web.

5.2 Hash collision attacks

Digests are only as strong as the hash function used to generate them. It is recommended that user agents refuse to support known-weak hashing functions and limit supported algorithms to those known to be collision resistant. Examples of hashing functions that are not recommended include MD5 and SHA-1. At the time of writing, SHA-384 is a good baseline.

Moreover, it is recommended that user agents re-evaluate their supported hash functions on a regular basis and deprecate support for those functions shown to be insecure. Over time, hash functions may be shown to be much weaker than expected and, in some cases, broken, so it is important that user agents stay aware of these developments.

5.3 Cross-origin data leakage

This specification requires the CORS settings attribute to be present on integrity-protected cross-origin requests. If that requirement were omitted, attackers could violate the same-origin policy and determine whether a cross-origin resource has certain content.

Attackers would attempt to load the resource with a known digest, and watch for load failures. If the load fails, the attacker could surmise that the response didn’t match the hash and thereby gain some insight into its contents. This might reveal, for example, whether or not a user is logged into a particular service.

Moreover, attackers could brute-force specific values in an otherwise static resource. Consider a JSON response that looks like this:

Example 8

{'status': 'authenticated', 'username': 'admin'}

An attacker could precompute hashes for the response with a variety of common usernames, and specify those hashes while repeatedly attempting to load the document. A successful load would confirm that the attacker has correctly guessed the username.